The present invention relates to an electronically commutatable motor, the stator field windings of which can be connected to a commutation frequency having a DC supply voltage to generate a rotating field for the permanent magnet rotor and can be disconnected from it. The field windings being switchable via semiconductor output stages which can be activated via control signals with the commutation frequency of a control unit and whose operating conditions change as a function of a specified or specifiable setpoint.
A control unit is generally connected to a motor of the above-mentioned type. The control unit activates the semiconductor switches of the semiconductor output stages with pulse-width-modulated control signals. The clock frequency of the clocked control signals is therefore in the high-frequency range while the commutation frequency of the control signals is a function of the design and speed of the motor and is thus substantially lower. The semiconductor output stages switch the applied DC supply voltage which, for example, is formed by the battery of the vehicle when such motors are used in motor vehicles. Changing the pulse width of the clock pulses of the clocked control signals controls the power and/or speed in such motors.
Various problems arise when such motors are used. Expensive semiconductor switches and driver circuits are required in the control unit and a high power dissipation occurs in it as well. This in turn results in an expense for cooling the semiconductors in the control unit. Since the control unit is directly coupled with the DC power supply, for example, the vehicle electrical system, high EMC interference signals are produced which necessitate elaborate interference suppression circuits. Various designs of the motor must be provided if DC supply voltages of different sizes are present. The expensive circuitry and the additional cooling expense make manufacture of the motor cost-intensive.
An object of the present invention is to provide electronically commutatable motor which avoids the disadvantages of the conventional pulse-width-modulated activation of the field windings without increased control and cooling expense.
According to the present invention, this object is achieved in that the DC supply voltage is supplied to a DC transformer whose output voltage for the semiconductor output stages with the field windings changes as a function of the setpoint and that the control unit completely trips the semiconductor output stages by force, continuously and independently of the setpoint, using unclocked control signals having the commutation frequency.
In this design of the motor, the commutation and the power or speed change are divided and performed separately. The control unit only takes over the commutation while the power or speed adjustment is taken over by a DC transformer, the output voltage of which changes as a function of a specified or specifiable setpoint. The semiconductor output stages with the field windings are maximally tripped by force, continuously, by the control unit, so that the output voltage of the DC transformer is responsible for the power or speed change.
This results in a number of advantages for the electronically commutatable motor according to the present invention. The high-frequency, pulse-width-modulated clock pulses of the control signals for the semiconductor output stages are eliminated. Slower semiconductor switches and simpler driver circuits can be used. Stepping up the vehicle power supply voltage via the DC transformer results in a reduction of the motor current and accordingly the power dissipation in the semiconductor switches at a given motor power. The semiconductor switches can therefore be operated without elaborate cooling. Since the control unit is decoupled from the vehicle power supply, i.e., the DC supply voltage, via the DC transformer and high-frequency clocking in the control unit is no longer necessary, the result is a low EMC interference level resulting in a lower expense for interference suppression. The reduction to the commutation function makes it possible to implement the control unit without a microcomputer, and therefore it can also be used at higher temperatures. The DC transformer can also be controlled without a microcomputer; only the electronic switch with a possible driver and an easily constructed control circuit in the DC transformer are required.
According to one embodiment, the DC transformer is designed in a conventional manner with a smoothing choke, a smoothing capacitor, a decoupling diode and an electronic switch, the switch being operated at a clock frequency and the pulse width of the switching pulses changing as a function of the setpoint. It is also possible to use DC transformers that are designed and controlled in another manner. The feedback control may be designed in such a way that a controller for deriving the pulse width of the switching pulses for the electronic switch is assigned to the DC transformer, the setpoint and the output voltage of the DC transformer being supplied to the controller.
The dependence of the output voltage of the DC transformer on the setpoint can be implemented in such a way that as the setpoint increases or decreases, the pulse width of the switching pulses of the electronic switch and accordingly the output voltage of the DC transformer increases or decreases.
The power and/or speed of the motor change as the output voltage of the DC transformer changes.
According to another embodiment, the DC transformer offers additional possible variations in that the output voltage is greater or less than the DC supply voltage and that the increase or decrease in the output voltage of the DC transformer is a function of the increase or the decrease in the setpoint.